A typical electrostatographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member to the image to be printed records an electrostatic latent image on the imaging member corresponding to the informational areas contained within the image to be printed. Generally, the electrostatic latent image is developed by bringing a toner or developer mixture into contact therewith.
One type of developer used in such printing machines is a liquid developer comprising a liquid carrier having toner particles dispersed therein. Generally, a suitable colorant also is present in the toner particles such as a dye or pigment. The liquid developer material is advanced into contact with the electrostatic latent image and the colored toner particles are deposited thereon in image configuration.
The developed toner image recorded on the imaging member may be transferred to an image receiving substrate such as paper via an intermediate transfer member. The toner image particles may be electrostatically transferred to the intermediate transport member by means of an electrical potential between the imaging member and the intermediate transfer member. After the toner image has been transferred to the intermediate transfer member, it is then transferred in image configuration to the image receiving substrate, such as by contacting the substrate with the image on the intermediate transfer member under heat and/or pressure.
In electrostatographic printing machines in which the toner image is electrostatically transferred by a potential between the imaging member and the intermediate transfer member, the transfer of the toner particles to the intermediate transfer member and the retention thereof should be as complete as possible so that the image ultimately transferred to the image receiving substrate will have a high resolution. Substantially 100% toner transfer occurs when most or all of the toner particles comprising the image are transferred and little residual toner remains on the surface from which the image was transferred. Substantially 100% toner transfer is particularly important when the imaging process involves generating full color images since undesirable shifting or color deterioration in the final colors can occur when the primary color images are not accurately and efficiently transferred to and from the intermediate transfer member.
Intermediate transfer members enable high throughput at modest process speeds. In color systems, the intermediate transfer member also improves registration of the final color toner image. In such color systems, component colors such as cyan, yellow, magenta, and black are synchronously developed onto one or more imaging members and transferred in registration onto an intermediate transfer member at one or more transfer stations. Intermediate transfer members also increase the range of final substrates that can be used, including papers, etc. A disadvantage of using an intermediate transfer member is that a plurality of transfer steps is required. In the process of electrostatically transferring toner images from the imaging member to an intermediate, charge exchange can occur between toner particles and the transfer member leading to less than complete toner transfer.
In a typical electrostatographic printing machine, toner particles, which can have either a negative or a positive charge, are positioned on the imaging member after development. A charged biased transfer roller or a corona supplies a charge to the backside of an intermediate transfer member to attract the oppositely charged toner particles. The charge on the backside of the intermediate transfer member, attracts the toner particles to the front side of the intermediate transfer member. Theoretically, toner transfer should be 100%. However, toner transfer is in practice frequently less than 100% due to the phenomenon of charge exchange between toner particles and the intermediate transfer member. In charge exchange, the toner particle charge is reduced so that it is not attracted to the transfer member or reversed so that it is repelled from the transfer member having a like charge. The more severe the charge exchange is, the less complete toner transfer to the intermediate is. Without wishing to be bound by any theory, charge exchange is believed to be associated with electrochemical phenomena occurring at the interface between the toner layer and the intermediate transfer member.
Charge exchange is a problem in that low charge, neutral, or `wrong sign` toner particles lead to incomplete toner transfer. The result is low resolution images on the image receiving substrate, including images suffering from image deterioration. If the images are color, the image additionally suffers from color shifting and color deterioration.
The formation of proper images also depends on the differences of the charge between the toner in the liquid developer and the electrostatic image to be developed. Thus, it is usually necessary to add a charge director compound to the liquid developer. Liquid developers containing charge directors provide images of good quality and resolution due to the improved charging of the toner. However, the use of charge directors also can exacerbate the problem of charge exchange between the toner and the intermediate transfer member.
U.S. Pat. No. 4,796,048 (Bean) discloses an apparatus which transfers a plurality of liquid images from a photoconductive member to a copy sheet. The apparatus may include an intermediate transport belt to transfer a toner image to a copy sheet with the use of a biased transfer roller. The intermediate transport belt has a smooth surface, is non-absorbent and has a low surface energy.
U.S. Pat. No. 4,708,460 (Langdon) discloses an intermediate transport belt that is preferably made from a somewhat electrically conductive silicone material having an electrical conductivity of about 10.sup.9 ohm-centimeters so that the belt is semiconductive.
U.S. Pat. No. 4,430,412 (Miwa et al.) discloses an intermediate transfer member, which may be a belt-type member that is pressed onto an outer periphery of a toner image retainer with a pressure roller. The intermediate member is formed with a laminate of a transfer layer comprising a heat resistant elastic body such as silicone rubber or fluororubber, and a heat resistant base material such as stainless steel. Silicone rubber is the only material shown in the examples as the transfer layer. No fluorocarbon elastomer having a reduced number of vulnerable sites as the transfer member surface is suggested.
U.S. Pat. No. 3,893,761 (Buchan et al.) discloses a xerographic heat and pressure transfer and fusing apparatus having an intermediate transfer member which has a smooth surface, a surface-free energy below 40 dynes per centimeter and a hardness from 3 to 70 durometer Shore .ANG.. The transfer member, preferably in the form of a belt, can be formed, for example, from a polyamide film substrate coated with 0.1-10 millimeters of silicone rubber or fluoroelastomer. Silicone rubber is the only material shown in the examples as the transfer layer. No fluorocarbon elastomer having a reduced number of vulnerable sites as the transfer member surface is suggested.
U.S. Pat. Nos. 4,684,238 (Till et al.) and 4,690,539 (Radulski et al.) disclose single layer intermediate transfer belts composed of polyethylene terephthalate or other suitable propylene material.
U.S. Pat. No. 5,119,140 (Berkes et al.) discloses a single layer intermediate transfer belt preferably fabricated from clear Tedlar.RTM. (a polyvinyl fluoride available from E. I. du Pont de Nemours & Co.), carbon loaded Tedlar.RTM. or pigmented Tedlar.RTM.. Tedlar.RTM. is a thermoplastic polymer, not an elastomer.
U.S. Pat. No. 5,099,286 (Nishise et al.) discloses an intermediate transfer belt comprising electrically conductive urethane rubber reportedly having a volume resistivity of 10.sup.3 to 10.sup.4 ohm-centimeter and a dielectric layer of polytetrafluoroethylene reportedly having a volume resistivity equal to or greater than 10.sup.14 ohm-centimeter.
U.S. Pat. No. 5,208,638 (Bujese et al.) relates to an intermediate transfer member surface comprising a fluorosilicone polymer with a conductive material dispersed therein upon a metal layer, which in turn is upon a dielectric layer. The use of fluorosilicone elastomers is disclosed, but there is no disclosure or suggestion of the improved transfer efficiency achievable by the use of fluorocarbon elastomer having a reduced number of vulnerable sites.
U.S. Pat. No. 5,233,396 (Simms et al.) discloses an intermediate transfer member which is semiconductive and comprises a thermally and electrically conductive substrate coated with a semiconductive, low surface energy elastomeric outer layer that is preferably Viton.RTM. B50 (a fluorocarbon elastomer).
U.S. Pat. No. 5,035,972 (EI-Sayed et al.) discloses an AB diblock copolymer as a charge director for negative electrostatic liquid developers.
A need remains for intermediate transfer members with sufficient mechanical strength and chemical and electrical properties that enable generation of high resolution images because of near complete transfer of all toner particles to and from the surface of the intermediate transfer member. Also, there is a need for a process of developing an image using an electrostatographic printing machine containing an intermediate transfer member and using a liquid color developer in which the resultant image has high color fidelity and resolution due to near complete transfer of the toner particles from the intermediate surface.